It has previously been proposed to sense the torque applied by electric screwdrivers and the like, hereinafter for short "electric tool" by measuring the motor current and utilizing the measured current as representative of torque being applied to a screw, nut, or the like (see German Patent Disclosure Document DE-OS No. 15 88 032). A shunt is placed in the motor current circuit, and voltage dropped across the shunt is sensed and applied to an electric control circuit. When the motor current reaches a certain value, resulting in a predetermined voltage drop across the shunt, a response circuit measures the voltage and controls the power circuit to the motor to interrupt current flow thereto. This system, while simple, has a disadvantage: if the motor is subjected to a substantial in-rush current pulse, the voltage across the shunt may be such that the turn-off circuit responds so that the tool can never be started. An additional disadvantage arises in some tools, particularly those in which the drive motor is subject to variations in characteristics due to tolerances in manufacture, or, for example, upon repair. Any change in the motor from a standard performance characteristic requires comparatively complex adjustment, since the relationship between motor current and torque may not be linear, and may not be consistent in many motors made under mass-production conditions. Readjustment of control circuitry or control elements, under mass-production conditions, is expensive and, preferably, to be avoided.
Various circuits have been proposed which include timers and which disconnect motor current after a predetermined operating time, corresponding, based on experience, to fastening a screwed element under predetermined conditions. Such circuits will not, however, provide uniform tightening torque to the screwed elements since the starting torque conditions of most screwed elements are not defined and thus the time required to secure screwed elements, with uniform torque, may vary.